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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Tungsten Flicker

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Deanan

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We've had a few questions lately from customers seeing flicker with tungsten sources especially at higher frame rates where you can't sync the shutter to 50/60Hz.

Tungsten sources can flicker because they aren't truly continuous sources.
Most tungsten sources don't appear to flicker because their decay from on to off is slow enough that it's not visible. With some sources (usually regular incandescent) the filament will decay faster and cause flicker (smaller filament and faster cooling).

HMI's with electronic ballasts, continuous (not pulsed) LEDs sources, or DC powered tungsten are the best ways to make sure you don't get flicker.

This side benefit is that HMI and LED sources balanced to daylight will provide a much nicer blue channel whereas with tungsten, you're under exposing blue significanly.

Deanan
 
I sure hope I don't notice halogen flicker at 24 frames. . . that would be a disaster.

Stephen
 
You should only notice this on very fine-filament sources. Household incandescent globes and maybe 150w or 250w proper lights... on a bad day.

The good news is that you can see it from the monitor... and larger sources [or sources with newer globes] shouldn't demonstrate this phenomenon.

And no, it's not a RED-specific thing. :)
 
I've certainly had this issue on highspeed shoots with Film, and the Phantom.

Is this enhanced more by the rolling shutter?
 
I've had it quite often on film on ramps and higher speeds and always use good tungsten sources or HMI's now. It happened quite easily on D20 also.

It has more to do with shutter angle and frame rate than anything else. I believe rolling shutter would not affect it's intensity just it's location. It's either going to be there or its not.
 
Unless you are shooting really, really high frame rates, it's a bit strange to be getting flicker problems from ordinary tungsten bulbs unless there is a power supply problem. I once filmed in a house with bad wiring and every time the owner used his xerox machine, all the lights in the house would flicker. And I had a small bulb over the stove flickering in some shots.

But that was ten years ago and haven't run into that problem again, ordinary tungsten lamps flickering at normal frame rates and shutter speeds. The decay time of a tungsten filament is usually long enough to not flicker except at high speeds, in which case, using bigger lamps with larger bulbs and filaments helps.
 
When i'm shooting I try to get directors to steer away from speeds like 36 and 75 etc - (this is PAL land) - because the Hertz hits at that frame rate hard (it's more likely to flicker as the frame rate isn't keeping pace with the peaks and troughs of the hertz output) - if you stay to 33.333, 50 or 100 (again PAL LAND here = don't do this in USA or death awaits you) - it just means you have less likelihood of problems.

In US off the top of my head 48 and 96 should be okay (and one or two intermediate speeds) - but check you are not using any small filament (low wattage) bulbs - they haven't enough heat to maintain a constant so there is reheat time and you get flicker. The ASC and Sammys manual have all the safe speeds. Plus I find Kino flos notorious for flicker (at 75) despite what it says on the box.

Just out of interest we shot a commercial on friday with the phantom and got flicker from 3 cinepar 12ks and 3 18ks (yes, they were all square wave) - it turned out to be the gas roiling in the cinepar bulbs becoming visible once you hit 2000 fps - we had 10,000 second shutter - stop on 100 ASA/1,50th was f256 :blink: - it really is the Ray Charles of cameras. (its also got about three stops of latitude).
 
yeah - I really have avoided 75fps for about ten years now. 50 or 100. (if you're in the US try 48 or 96 - they should be good but check the ASC or Sammy's manuals) - I've had too much wine to do the maths.:biggrin:
 
For a rolling shutter digital sensor, flicker effect on footage is not dependent on frame rate directly. It is dependent on the exposure time. For continuous sinusoids, if the exposure time is an integer multiple of reciprocal of flicker frequency (sometimes it is twice the line frequency), then there should not be any visible artifacts on footage, and hence, the notion of "flicker free".

Flicker is maximum when exposure time is an odd multiple of reciprocal of 4 * line frequency for sinusoids.

What flicker-free essentially means is that the AC harmonics die off. The DC component is still there and depends upon certain fixed parameters.
 
Do you use that line to pick up chicks? :sarcasm:

Seriously though - exposure time is the core problem across the board, not limited exclusively to digital (although rolling shutter may introduce other problems) - Frame rate effects exposure time, no? So when the camera is receiving different exposures on a per frame basis you get flicker but exposure time in a motion picture camera is limited by frame rate (with some minor control through shutter) - bottom line, I find, is to, whever possible, marry the two (frame rate and hertz) whenever possible which, lazily, means sticking in multiples of 24/25 (depending on AC supply locally) - but whatever works for you personally. I haven't had a flicker problem on a shoot for about eight years (fingers crossed)

or we could just all go back to DC brute arcs - I loved those things.
 
In theory exposure time is only bounded by the framerate, and therefore, as long as your exposure time is less than the 1 / framerate, exposure time may be treated independent of framerate. In film camera exposure time is controlled by shutter angle, where as in digital cameras, exact time can be specified. Therefore, many digital cameras may provide finer control over exposure time compared to film camera.

I see many people saying that "faster framerate" will help here -- which it does sometimes, but not inherently because of a faster framerate, but a faster framerate might force the exposure time to even a lower number to accommodate a faster framerate. But, as I said, it is not the framerate directly, it is the exposure time that matters.

For the same frame rate you can have "flicker free" footage if exposure time is n * 1 / (2 * line freq), and worst flicker for (2n + 1) / (4 * line freq) -- i.e., even for the same framerate, you can have flicker varying from none to max depending upon what was the exposure time.
 
I wonder if this will be a problem with Dedo lights? They are DC powered.
 
Sophisticated algorithms exist for correcting flicker and other exposure problems. I do not know if Red is incorporating them in-camera or in software when using RedCine/RedAlert/Scratch/etc.
 
Evin Grant said:
Another thing I noticed is that using 1/60th or 1/120th shutter in normal mode reduced the flicker to an almost imperceptible level. This makes sense here in the US where the juice is 60hz.
Click to expand...

Exactly, as I said that for "flicker free" exposure time should be:

n * 1 / (2 * line freq),

and line freq in US is 60 Hz, so "flicker free" is

n / 120, i.e., 1/120, 2/120, 3/120, 4/120, 5/120, but not 6/120, as that is more than 1/24 (the limit imposed by 24 fps framerate).
 
Street light strobe

Street light strobe

We did some tests last night, under street light, and strobed like crazy, even at 24fps. We're shooting several street scenes here in a couple of weeks, so we'll need to black out all the street lights and light from scratch. (I always planned to light from scratch, but didn't realise we'd have to black out like this.) No idea what kind of light-source is in an Australian street lamp, but this is probably going to be an ongoing issue for us.
 
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